Motor vehicle power train

ABSTRACT

A motor vehicle power train includes an engine (26) mounted longitudinally near the front of the vehicle and a transmission assembly (35) mounted intermediate front and rear differentials (17, 18). The transmission assembly is connected to the engine through an input propshaft (31), to the front differential through a front propshaft (37) and to the rear differential through a rear propshaft (38). The transmission assembly comprises a change speed transmission (28) which receives drive from the input propshaft and a hollow output shaft (49), a transfer transmission comprising a center differential (30) which is drivably connected to the hollow output shaft, has a front differential output shaft (54) for transmitting drive to the front propshaft and a rear differential output shaft (56) for transmitting drive to the rear propshaft, one of the differential output shafts (54) extending co-axially through the hollow output shaft. The transmission assembly also comprises an auxiliary output transmission (39) in the drive from the front differential output shaft to the front propshaft, the transmission assembly being arranged so that front and rear rotary outputs (37B, 38B) to the front and rear propshafts are axially offset from a rotary input (33) from the input propshaft, the front rotary output by an amount greater than that of the rear rotary output. The change speed transmission is conveniently adapted from a known automatic transmission for front wheel drive vehicles and can be connected using an input transmission (34).

The invention relates to motor vehicle power trains and is particularlyconcerned with power trains of four wheel drive vehicles of the typewhich have a longitudinally mounted engine.

Known power trains of conventional four wheel drive vehicles typicallycomprise a change speed transmission mounted axially in line with theengine and a transfer transmission which is mounted directly on thechange speed transmission to drive front and rear differentials throughpropshafts offset from the engine and the change speed transmission.Alternatively, a separate transfer transmission is spaced from thechange speed transmission and connected to it through a shortuniversally jointed propshaft.

Packaging of power trains in motor vehicles is of increasing importanceparticularly as vehicles become more compact and available spacedecreases. This creates a conflict for the vehicle designer betweenproviding adequate accommodation for the driver and passengers andmounting the power train components in an arrangement most appropriatefor their function of driving the vehicle.

A power train is described in FR-A-2 695 880 which shows a transmissionwhich is intended for a sports car and the object is to move the weightof the transmission assembly close to the rear wheels so that there is abalanced distribution of load between the two axles of the vehicle. Tofulfil this object of weight distribution, FR-A-2 695 880 couples thetransmission assembly directly to the rear differential casing so thatthe rear output from the center differential is taken directly by thebevel pinion shaft. This causes more problems in providing adequateaccommodation for the passengers and luggage and adds to the polarmoment of inertia of the vehicle, affecting ride and handling.

An object of the present invention is to provide a motor vehicle powertrain which helps to optimize the available space without any of thedisadvantages outlined above.

According to one aspect of the invention a power train for a motorvehicle is characterised in that the transmission assembly is separatefrom the rear differential and placed at or near the center of thevehicle and is spaced from the rear differential by a rear propshaftwhich drivably connects the rear differential to the rear rotary output.

The change speed transmission may be coaxial with the hollow outputshaft, an input transmission being adapted to transmit drive from thedrive input to the input shaft of the change speed transmission, inwhich case the input transmission and the auxiliary output transmissionare conveniently arranged at the same end of the transmission assembly.This arrangement is particularly suitable where the change speedtransmission is an epicyclic type automatic transmission or a toroidaltype CVT.

Alternatively, the change speed transmission may be a layshaft typehaving an input shaft coaxial with the rotary input and axially offsetfrom the hollow output shaft. This arrangement is particularly suitablewhere the change speed transmission is a constant mesh type, e.g.sychromesh or is a belt and pulley type CVT.

The transfer transmission may include an epicyclic final drive having asun gear connected to the hollow output shaft and a planet gear carrierfor transmitting drive to the center differential, in which case theepicyclic final drive may provide a plurality of selectable ratios.

In one arrangement according to the invention the transmission assemblyis spaced from the engine, the rotary input being connected to theengine by an input propshaft, in which case a drive coupling (e.g.torque converter or friction clutch) in the drive between the engine andthe transmission assembly can be mounted on the engine so as to beoperable to transmit drive to the transmission assembly through theinput propshaft. A bell housing for the drive coupling can be mounted onthe engine and, as an alternative to independent mounting of the engineand the transmission assembly, the bell housing and the transmissionassembly can be connected by a tubular housing which surrounds the inputpropshaft.

Alternatively, and especially conveniently where the change speedtransmission is an automatic epicyclic type, the transmission assemblycan comprise a drive coupling (particularly a torque converter) in thedrive between the engine and the change speed transmission. In thatcase, where the drive coupling is a torque converter a pump for thechange speed transmission is conveniently located on the opposite sideof the torque converter to the engine.

The transmission assembly may in use be mounted substantially midwaybetween the front and rear differentials. This helps to reduce the polarmoment of inertia of the vehicle and improves handling.

The front and rear propshafts can be of substantially equal length. Thisis particularly advantageous where rigid axles are used and both of thefront and rear differentials form part of the unsprung weight of thevehicle.

Advantageously, the change speed transmission includes rotary drivecomponents and selector components substantially identical to thoseproduced for a change speed transmission used in a front wheel drivemotor vehicle. This helps to reduce tooling costs and development time.Similarly, the change speed transmission may include a casing componentsubstantially identical to one produced for a change speed transmissionused in a front wheel drive motor vehicle.

Also novel and according to another aspect of the invention is a motorvehicle which incorporates a power train according to said one aspect ofthe invention. In such a motor vehicle, the transmission assembly can behoused partly within a tunnel or cavity in a floor of the motor vehicle.This minimises the effect of any intrusion into the passengercompartment. The tunnel or cavity may project upwardly between the footwells for front and rear passengers, again minimising the effect of anyintrusion into the passenger compartment.

The invention will now be described by way of example with reference tothe accompanying drawings in which:

FIG. 1 is a diagrammatic side elevation of a motor vehicle incorporatinga power train in accordance with the invention;

FIG. 2 is a plan view of the vehicle shown in FIG. 1 showing the powertrain and wheels of the vehicle;

FIG. 3 is a cross-section of part of the drive train of an existingfront wheel drive vehicle incorporating an automatic transmissionassembly used for a front wheel drive vehicle and which is modified foruse in the present invention;

FIG. 4 is a longitudinal cross-section similar to FIG. 3 showing a firststage of modification;

FIG. 5 is a diagrammatic view of the transmission assembly shown in FIG.4 showing a final stage of modification for use in the presentinvention;

FIG. 6 is a cross-sectional view of the right hand end of thetransmission assembly shown in FIG. 5 shown partly in diagrammatic formand illustrating a transfer transmission in more detail;

FIG. 7 is a diagrammatic cross-section showing a power train inaccordance with the invention using a synchromesh type of change speedtransmission;

FIG. 8 is a diagrammatic cross-section through part of the power trainshown in FIG. 7, generally on the line VIII--VIII;

FIG. 9 is a longitudinal cross section of a transmission assemblysomewhat similar to that shown in FIG. 4 but showing the transmissionassembly mounted on a housing of a torque converter; and

FIG. 10 is a plan view of a vehicle having the transmission assemblyshown in FIG. 9.

In FIGS. 1 and 2, a motor vehicle 11 includes a pair of front wheels 12,13 and a pair of rear wheels 14, 15. The front wheels 12, 13 are carriedby a front axle 16 having a differential unit 17. Similarly, the rearwheels 14, 15 are carried by a rear axle 18 having a differential unit19. The front axle 16 is suspended from a vehicle chassis 21 by means ofleading links 22 and coil springs 23. The rear axle 18 is suspended fromthe chassis 21 by means of trailing links 24 and air springs 25.

The front and rear axles 16, 18 form part of a power train which alsoincludes an engine 26, a bell housing 27 which covers a torque converterattached to a crankshaft of the engine 26 and a transmission assembly 35mounted spaced from and independently of the engine 26. The transmissionassembly 35 is drivably connected to the engine 26 by an input propshaft31 having universal joints 32 and 33 arranged one each end, the rearmostjoint 33 acting as a rotary input for the transmission assembly 35. Thetransmission assembly 35 includes an automatic change speed transmission28 and a transfer transmission 30, drive from the input propshaft 31being transmitted to the change speed transmission 28 through an inputtransmission 34 which will be described in more detail below. Thetransfer transmission 30 is drivingly connected to the frontdifferential 17 by a front propshaft 37 having universal joints 37A and37B and to the rear differential 19 through a rear propshaft 38 havinguniversal joints 38A and 38B, the rearmost joint 37B of the frontpropshaft 37 acting as a front rotary output for the transmissionassembly 35 and the front joint 38B of the rear propshaft 38 acting as arear rotary output for the transmission assembly. Drive from thetransfer transmission 30 to the front propshaft 37 is transmittedthrough an output transmission 39 which is also described in more detailbelow. It can be seen that the rotary input (universal joint 33) and thefront and rear rotary outputs (universal joints 37B and 38B) arestaggered or axially offset, i.e. their axes are offset from each other,the offset between the rotary input and the front rotary output beinggreater than that between the rotary input and the rear rotary output.

The drive train of the existing transmission assembly 35A of a frontwheel drive vehicle as shown in FIG. 3 comprises a bell housing 40 whichis normally mounted on to an engine 42 and houses a torque converter 43.A housing 45 for a valve block 46 and a pump 47 is also mounted on thebell housing 40, the pump 47 receiving drive from the engine 42 in knownmanner. Drive from the torque converter 43 is transmitted by a drivingsprocket 60A, a chain 44 and a driven sprocket 62A to the hollow inputshaft 61 of an automatic change speed transmission 28A which contains aseries of epicyclic gear trains, clutches and brakes which enable driveratios for the vehicle to be selected in known manner. The change speedtransmission 28A includes a hollow output shaft 49.

The transmission assembly 35A has an epicyclic final drive 29A whichgives a required final drive ratio and transmits drive to a differential52. A shaft 54 extends from one side of the differential 52 coaxiallythrough the hollow output shaft 49 and other parts of the automaticchange speed transmission 28A for driving a front wheel on one side ofthe vehicle and another shaft 56 extends from the other side of thedifferential for driving a front wheel on the other side of the vehicle.

The transmission assembly shown in FIG. 3 is modified as described belowto form part of a power train in accordance with the present invention.The term "modification" will be understood to include designmodification (for series production) as well as modification of actualcomponents (for feasibility prototypes and very low volume production).In either case a number of rotary drive components and selectorcomponents (e.g. clutches and brakes) can be substantially identical tothose used in the FIG. 3 transmission and it may also be possible to usecasing components in this way.

FIG. 4 shows a first stage in the modification in which the torqueconverter 43 has been removed and an end cover 58 put in its place. Atorque converter 43A is instead housed within the bell housing 27 andtransmits drive through the input propshaft 31 so as to drive the pump47 and the automatic change speed transmission 28B. The torque converter43A is connected to the pump 47 and valve block 46 by pipes (not shown)and a muff coupling for operation of the usual lock-up clutch in thetorque converter. The pump 47 may alternatively be replaced by anotherpump (not shown) driven directly by the engine or supplemented by anauxiliary engine driven pump.

Drive to the automatic change speed transmission 28B is transmittedthrough an input transmission 34A which comprises the driving sprocket60A, the driven sprocket 62A and the chain 44. The shaft 54 is used toconnect to the front propshaft 37 and the shaft 56 used to connect tothe rear propshaft 38, the epicyclic final drive 29A and thedifferential 52 serving as a transfer transmission in which thedifferential 52 acts as a center differential to allow variationsbetween the speeds of the front and rear wheels 12, 13, 14, 15.

FIG. 5 shows diagrammatically the final stage of modification, thisbeing the stage shown in the power train in FIGS. 1 and 2. The shaft 54carries a gear 64 which meshes with a gear 66 drivably connected to thefront propshaft 37 in substantially a 1:1 ratio, the gears 64, 66providing the auxiliary output transmission 39. The shaft 56 driven bythe differential 52 is drivably connected to the rear propshaft 38. Itwill be noted that the input transmission 34 and the output transmission39 are arranged at the same end of the change speed transmission 28 andthat instead of the chain 44 and sprockets 60A, 62A, the inputtransmission comprises meshing spur gears 60, 62.

FIG. 6 shows in some detail the finally modified transfer transmission30, particularly the epicyclic final drive 29.

In FIG. 6, the hollow output shaft 49 of the change speed transmission28 has a portion 70 which is connected through splines 72 to a sun gear74. The sun gear 74 meshes with a series of planet gears 76 which aremounted for rotation on pins 78 of a planet gear carrier 80. The planetgears 76 also mesh with an annulus gear 82 fixed to a casing 84 of thechange speed transmission 28. The planet gear carrier 80 has an annularextension 86 formed with a circumferential row of axial splines 88which, for convenience of manufacture, are identical to the splines 72.A selector member 90 is slidably mounted on splines 92 formed on a drivemember 94 of the differential 52. The selector member 90 is formed witha circumferential row of splines 96 which can be slid optionally intoengagement with the splines 72 on the sun gear 74 or with the splines 88on the planet gear carrier 80. The drive member 94 is mounted forrotation in a bearing 98 on the housing 84. The differential 52 is ofknown kind and may be a limited slip type, e.g. a Torsen (Trade Mark). Asuitable selector mechanism 100, part of which is shown in FIG. 6, isprovided for shifting the selector member 90. Operation of the epicyclicfinal drive 29 will now be described.

With the transmission components stationary, drive in high range to theshafts 54, 56 is provided by shifting the selector member 90 tointerengage the splines 72 and 96, drive from the hollow output shaft 49being transmitted directly to the drive member 94. To provide a lowrange, the selector member 90 is shifted so as to interengage thesplines 88 of the planet gear carrier 80 with the splines 96 of theselector member 90. In that way, drive from the hollow output shaft 49will be transmitted through the planetary gearing and out through theplanet gear carrier 80 via the selector member 90 so as to rotate thedrive member 94 at a reduction ratio.

The epicyclic final drive 29 is readily adaptable from a known epicyclictransmission already used to provide high and low ranges on a four wheeldrive vehicle.

In normal use of the vehicle, the auxiliary output transmission 39applies torque to the front propshaft 37 in a direction which isopposite to that applied to the rear propshaft 38, i.e. they normallyrotate in opposite directions. That is advantageous in that torquereaction from the front and rear propshafts 37, 38 is thereby cancelled.However, the auxiliary output transmission 39 could be arranged so thatthe front propshaft 37 to rotate in the same direction as the rearpropshaft 38, e.g. by using sprockets and a chain similar to the inputtransmission 34A in FIG. 4 and modifying the gearing of the frontdifferential 17.

Reference is now made to FIG. 7 which shows a modified transmissionassembly 35C in which a synchromesh type of change speed transmission110 is used instead of the epicyclic type automatic change speedtransmission 28. Synchromesh transmissions are normally manuallyoperated but, as is now well known, can be adapted for automatic orsemi-automatic operation using servo-actuators. Parts in FIG. 7 whichcorrespond to parts shown in FIGS. 1 to 6 carry the same referencenumerals and will be not be described in detail.

In FIG. 7, the engine 26 transmits drive via a clutch (not shown) in thebell housing 27 and an input propshaft 31 having universal joints 32 and33 arranged on each end. Rotating components of the change speedtransmission 110 include an input shaft 112 and a hollow output shaft114.

The change speed transmission 110 comprises first, second, third, fourthand fifth forward ratio constant mesh gear trains 116, 117, 118, 119 and120 respectively and a reverse gear train 122. The reverse ratio isengaged by moving a reverse idler gear (not shown) into mesh. Theforward ratios are selected by operating synchromesh clutches by meansof a conventional selector mechanism (not shown). The hollow outputshaft 114 acts as an input shaft for the transfer transmission 30 whichmay be similar to the transmission 30 shown in detail in FIG. 6. As inFIG. 6, the transfer transmission 30 includes a center differential 52from which drive is transmitted to the front propshaft 37 via a shaft 54passing through the hollow output shaft 114 and an auxiliary outputtransmission 39 comprising gear wheels 64, 66 as in FIG. 5. The centerdifferential 52 also transmits drive to the rear propshaft 38.

The synchromesh type change speed transmission 110A can conveniently besubstituted by other layshaft type transmissions, including constantmesh types with dog clutches or multiple friction clutches orcontinuously variable transmissions (CVTs) of the belt and pulley type.Similarly, the epicyclic type of automatic change speed transmission 28can conveniently be replaced by a CVT of a type having concentric inputand output shafts, e.g. a toroidal type such as the Torotrak (TradeMark), modified as necessary to have a hollow output shaft and a hollowinput shaft.

The transmission assembly 35 or 35C is preferably positionedlongitudinally of the power train so that the front and rear propshafts37, 38 are of substantially equal length. In that way, vibration andharshness caused or exacerbated by the use of excessively shortpropshafts is reduced and the service life of the associated universaljoints can be increased. Moreover, by positioning the transmissionassembly 35 or 35C substantially mid way between the axles 16 and 18,the polar moment of inertia of the vehicle can be reduced and thevarious components of the transmission can be packaged to provide goodground clearance without undue difficulty. In that respect, reference isagain made to FIG. 1.

Looking at FIG. 1, it will be noted that by placing the transmissionassembly 35 or 35C at the mid-point of the vehicle, a tunnel or cavity124 formed in a floor 126 for receiving the auxiliary input transmission34 of the automatic change speed transmission will be positioned wellforward of a rear passenger foot well 128 and will not intrude into afront passenger foot well 130. That feature is particularly advantageousfrom the point of view of passenger leg room and comfort. Also, theauxiliary output transmission 39 can be positioned outside a groundclearance curve indicated at 131 which is most advantageous from thepoint of view of maintaining optimum ground clearance for the vehicle.The synchromesh transmission 110 of FIG. 7 is shown in broken outlineand it will be seen that a cavity or tunnel 124a needed to clear thetransmission 110 does not intrude into the footwells 128, 130.

By spacing the transmission assembly 35 or 35C from the engine 26 andbell housing 27, there is greater space available for the packaging ofengine auxiliaries, particularly exhaust components. In particular, acatalytic converter can be positioned close to the engine 26, i.e. withonly a short intermediate length of pipe, so that it can reach itsoperational temperature much sooner than if it is positioned furtherfrom the engine. A further advantage is that mountings for the engine 26are required to absorb engine torque only. However, in a modification,the bell housing 27 and the casing of the transmission assembly 35 or35C are connected by a tubular housing 36 (shown in dashed lines in FIG.2) which surrounds the input propshaft 31. This eliminates the need foruniversal joints 32, 33. The staggering of the front and rear propshafts37, 38 as will be appreciated from FIGS. 2 and 5 is also useful from thepoint of packaging.

Instead of positioning the torque converter 43A within the bell housing27, it may be incorporated in the transmission assembly 35A as shown inFIG. 3 with the casing adapted accordingly. Drive from the engine 26will be transmitted to the torque converter 43 through the inputpropshaft 31 and a shaft concentric with the pump 47. Alternatively, thetorque converter and the pump and valve block can be transposed so thatthe pump and valve block are on the opposite side of the torqueconverter to the engine.

In FIG. 5, the input transmission 34 comprises meshing gears 60, 62.Instead of using meshing gears, it is envisaged that a chain drive maybe used provided that the transmission 28 will finally provide a driveoutput in the correct rotational direction for the vehicle.

Reference is now made to FIGS. 9 and 10 in which parts corresponding toparts shown in FIGS. 1 to 6 carry the same reference numerals with theaddition of 200 and are not described again in detail.

In FIGS. 9 and 10, the transmission assembly 235 is mounted on a bellhousing 227 for a torque converter 243 instead of being spaced therefromand connected thereto by the input propshaft 31. The torque converter243 drives a tubular input shaft 233 which acts as the rotary input tothe transmission assembly, the pump 247 being engine driven through aconcentric shaft 250. First and second meshing gears 260, 262 providethe input transmission to the change speed transmission 228. As before,the automatic change speed transmission 228 contains a series of geartrains clutches and brakes to enable drive ratios for the vehicle to beselected in known manner. The change speed transmission 228 transmitsdrive through a hollow output shaft 249 as FIG. 4 to an epicyclictransfer transmission 229 which in turn transmits drive to adifferential 252. The differential 252 drives a shaft 254 extendingcoaxially through the hollow output shaft 249 and through the remainderof the change speed transmission 228 for driving a front differential217 via gears 264, 265 and a front propshaft 237. The differential 252also drives a shaft 256 which transmits drive to a rear differential 219via a rear propshaft 238. The front differential 217 transmits drivethrough front driveshafts 230 for driving front wheels 22, 23 and therear differential 29 transmits drive through rear driveshafts 232 fordriving rear wheels 214, 215. The front and rear differentials 217, 219are mounted on a body or chassis of the vehicle and the front and rearwheels 212, 213, 214 and 215 are carried by respective independentsuspensions 212a, 213a, 214a and 215a. It will be noted from FIG. 10that the front propshaft 237 is shorter than the rear propshaft 238,this not being a disadvantage in a vehicle having independent suspensionwhere the front and rear differentials are mounted on the body orchassis of the vehicle. If desired, the meshing gears 260, 262 and 264,265 can be replaced by respective chain drive arrangements as describedabove with respect to FIG. 5.

For packaging convenience, the input propshaft 31 or the tubular inputshaft 233 and the front and rear propshafts 37 or 237, 38 or 238 willnormally be disposed somewhat as shown in FIG. 8, this furtherillustrating the staggering or axial offset between the rotary input andthe front and rear rotary outputs and showing that the front rotaryoutput to the front propshaft 37 or 237 is offset by a greater amountthan that of the rear rotary output to the rear propshaft 38 or 238.

Depending upon the design of the particular vehicle in which the powertrain is to be installed, the outputs may be disposed to the other sideof the vehicle, as illustrated in FIG. 8 by an alternative frontpropshaft 37C, shown dotted. Also, instead of the center differential 52or 252 could be at the same end of the transmission assembly as theauxiliary output transmission 39, 239 so that the rear differentialoutput shaft 56 or 236 extends through the hollow output shaft 49 or114.

We claim:
 1. In a motor vehicle having a pair of front wheels and a pairof rear wheels, a power train comprising:an engine which, in use, ismounted longitudinally in the vehicle adjacent a front end thereof; afront differential disposed, in use, between the front wheels anddrivably connected thereto; a rear differential disposed, in use,between the rear wheels and drivably connected thereto; a transmissionassembly mounted intermediate and spaced from the front and reardifferentials, the transmission assembly having a rotary input drivablyconnected to the engine, a front rotary output drivably connected to thefront differential and a rear rotary output drivably connected to therear differential; a front propshaft drivably connecting the frontrotary output to the front differential; and a rear propshaft drivablyconnecting the rear rotary output to the rear differential; saidtransmission assembly comprising;a change speed transmission having aninput shaft for receiving drive from the rotary input and a hollowoutput shaft; a transfer transmission comprising a center differentialwhich is drivably connected to the hollow output shaft, has a frontdifferential output shaft for transmitting drive to the front rotaryoutput and a rear differential output shaft for transmitting drive tothe rear rotary output, whereby one of the differential output shaftsextends co-axially through the hollow output shaft; an auxiliary outputtransmission in the drive from the front differential output shaft tothe front rotary output; and the transmission assembly being arranged sothat the front and rear rotary outputs are axially offset from therotary input, the offset of the front rotary output being greater thanthat of the rear rotary output.
 2. The power train according to claim 1,in which the change speed transmission is co-axial with the hollowoutput shaft, the transmission assembly further comprising an inputtransmission which is adapted to transmit drive from the drive input tothe input shaft of the change speed transmission.
 3. The power trainaccording to claim 2, in which the input transmission and the auxiliaryoutput transmission are arranged at the same end of the transmissionassembly.
 4. The power train according to claim 2, in which the changespeed transmission is an epicyclic automatic transmission.
 5. The powertrain according to claim 1, in which the change speed transmission is alayshaft transmission having an input shaft co-axial with the rotaryinput and axially offset from the hollow output shaft.
 6. The powertrain according to claim 5, in which the change speed transmission is aconstant mesh transmission.
 7. The power train according to claim 1, inwhich the transfer transmission includes an epicyclic final drive havinga sun gear connected to the hollow output shaft and a planet gearcarrier for transmitting drive to the center differential.
 8. The powertrain according to claim 7, in which the epicyclic final drive providesa plurality of selectable ratios.
 9. The power train according to claim1 and further comprising an input propshaft and wherein the transmissionassembly is spaced from the engine and the rotary input is connected tothe engine by the input propshaft.
 10. The power train according toclaim 9, and further comprising a drive coupling in the drive betweenthe engine and the transmission assembly, the drive coupling beingmounted on the engine so as to be operable to transmit drive to thetransmission assembly through the input propshaft.
 11. The power trainaccording to claim 10 and further comprising a bell housing on theengine and in which the drive coupling is mounted and a tubular housingwhich surrounds the input propshaft and connects the bell housing to thetransmission assembly.
 12. The power train according to claim 9, inwhich the transmission assembly further comprises a drive coupling inthe drive between the engine and the change speed transmission.
 13. Thepower train according to claim 12, in which the drive coupling is atorque converter and the transmission assembly further comprises a pumpfor the change speed transmission and which is located on the oppositeside of the torque converter to the engine.
 14. The power trainaccording to claim 1, in which the transmission assembly is in usemounted substantially midway between the front and rear differentials.15. The power train according to claim 1, in which the front and rearpropshafts are of substantially equal length.
 16. The power trainaccording to claim 1, in which the change speed transmission includesrotary drive components and selector components substantially identicalto those produced for a change speed transmission used in a front wheeldrive motor vehicle.
 17. The power train according to claim 16, in whichthe change speed transmission includes a casing component substantiallyidentical to one produced for a change speed transmission used in afront wheel drive motor vehicle.
 18. A motor vehicle having a pair offront wheels, a pair of rear wheels and a power train comprising:anengine which, in use, is mounted longitudinally in the vehicle adjacenta front end thereof; a front differential disposed, in use, between thefront wheels and drivably connected thereto; a rear differentialdisposed, in use, between the rear wheels and drivably connectedthereto; a transmission assembly mounted intermediate and spaced fromthe front and rear differentials, the transmission assembly having arotary input drivably connected to the engine a front rotary outputdrivably connected to the front differential and a rear rotary outputdrivably connected to the rear differential; a front propshaft drivablyconnecting the front rotary output to the front differential; and a rearpropshaft drivably connecting the rear rotary output to the reardifferential; said transmission assembly comprising:a change speedtransmission having an input shaft for receiving drive from the rotaryinput and a hollow output shaft; a transfer transmission comprising acenter differential which is drivably connected to the hollow outputshaft, has a front differential output shaft for transmitting drive tothe front rotary output and a rear differential output shaft fortransmitting drive to the rear rotary output, whereby one of thedifferential output shafts extends co-axially through the hollow outputshaft; an auxiliary output transmission in the drive from the frontdifferential output shaft to the front rotary output; and thetransmission assembly being arranged so that the front and rear rotaryoutputs are axially offset form the rotary input, the offset of thefront rotary output being greater than that of the rear rotary output.19. The motor vehicle according to claim 18 and further comprising afloor which defines a tunnel in which the transmission assembly ispartly housed.
 20. The motor vehicle according to claim 19, in which thefloor defines footwells for front and rear passengers and the tunnelprojects upwardly between the footwells.